Lightweight metal bipolar plates and methods for making the same

ABSTRACT

Thin, light weight bipolar plates for use in electrochemical cells are rapidly, and inexpensively manufactured in mass production by die casting, stamping or other well known methods for fabricating magnesium or aluminum parts. The use of a light metal, such as magnesium or aluminum minimizes weight and simultaneously improves both electrical and thermal conductivity compared to conventional carbon parts. For service in electrochemical cells these components must be protected from corrosion. This is accomplished by plating the surface of the light weight metal parts with a layer of denser, but more noble metal. The protective metal layer is deposited in one of several ways. One of these is deposition from an aqueous solution by either electroless means, electrolytic means, or a combination of the two. Another is deposition by electrolytic means from a non-aqueous solution, such as a molten salt.

FIELD OF THE INVENTION

The present invention relates to bipolar plates between adjacentelectrochemical cells. More particularly, the invention relates tolightweight bipolar plates and methods for their construction.

BACKGROUND OF THE INVENTION

Most of the components currently used in proton exchange membrane (PEM)fuel cells are derived from designs originally developed for use inphosphoric acid fuel cells (PAFC), and are not optimal for the higherperformance of PEM fuel cells.

By the mid-70s, components consisting entirely of carbon were made foruse in PAFC's operating at temperatures in the 165-185° C. range. Oneparticular manufacturer has made bipolar plates by molding a mixture ofgraphite powder (approximately 67 wt %) and phenolic resin(approximately 33 wt %) and carefully heat-treating to carbonize theresin without introducing excessive porosity by rapid degassing.Typically, heat treatment to 900° C. was sufficient to give the requiredchemical, physical and mechanical properties. The bipolar plates weremolded flat and were machined to produce the required fluid distributionor collection grooves (or cooling grooves for the bipolar plate).Somewhat later in time, grooved plates were molded in a die (which wasslightly oversized to compensate for shrinkage during baking) to producea glassy graphitic, carbon-composite plate. However, whilecarbon/graphite bipolar plates are effective, they are expensive and,because it is difficult to produce thin carbon based bipolar plates,stacks built with these plates tend to be heavy and bulky.

One alternative for overcoming these limitations is to use a moldablegraphite-based composite that does not have to be carbonized. Graphitepowder, which serves as the conductor, is bonded into a rigid piece witha polymer matrix. The graphite retains its conductivity and corrosionresistance, and the polymer binder, which must also be stable under PEMoperating conditions, allows the plate to be formed by conventionalpolymer forming processes. This approach has distinct limitations. Whenthe graphite is diluted with the polymer, its conductivity, alreadylower than any metal, is reduced even further. A seven kilowatt stackwith pure graphite bipolar plates would be expected to have a 16 Wattinternal resistive loss. When the graphite is dispersed in a polymermatrix, this loss will be larger.

Yet another example of a bipolar plate is a solid titanium metal sheet.The titanium is resistant to corrosion in many applications, providesgreater electronic conductivity than does graphite, and can be made inrelatively thin sheets. However, titanium is very expensive andrelatively heavy itself.

Therefore, there is a need for a lightweight bipolar plate that providesthe desired conductivity and can withstand the corrosive environmentstypically found in fuel cells and the like.

SUMMARY OF THE INVENTION

The present invention relates to bipolar separators or plates positionedbetween adjacent electrochemical cells, including lightweight bipolarplates and methods for their construction. The invention involvesmethods for depositing metals onto Mg and Al plates from either anaqueous solution or a non-aqueous solution of molten salts.

In one embodiment of the invention, Pd—Ni alloys can be electroplatedfrom an aqueous solution onto electroless Ni deposits using directcurrent (DC), pulsed current (PC), and pulse reversal (PR) platingconditions. Using simple PC plating conditions, the size ofelectrodeposited Ni crystals were significantly smaller than thoseobtained using DC plating, thereby, reducing porosity and improving thehardness of the coating. Pulse reversal plating yielded a Ni coatingwith improved corrosion resistance in various environments, because themost active metal crystals are continuously dissolved during the anodicpulse.

Because of the hydrophilic nature of gas flow channels in metal bipolarplates, the channels may flood with water and hinder the supply ofgaseous reactants to the electrodes. Therefore, the surfaces of gas flowchannels are preferably hydrophobic to prevent the attachment of waterdroplets. In order to make the channels hydrophobic, an electroplatedmetal-PTFE composite coating call be deposited on the surface of thePd—Ni coating. The unique water repellent property of PTFE particlesincorporated into the composite coating will prevent the penetration ofliquid water into any pores in the deposited layer which will also aidin preventing corrosion of the metal substrate. The electroplated metalused in the composite coating can be selected from Ru, Pd, and Au.

In a second embodiment of the invention, a non-aqueous plating system,such as a molten salt solution, is used to plate metal onto thesubstrate. One difficulty associated with an aqueous system for platingan Al or Mg substrate is that the Al and Mg react with water to form anoxide film. Thus, the use of an aqueous plating system requires amulti-step, multi-layer plating process to remove the oxide film andplate the desired metals onto the substrate. By contrast, a non-aqueoussolvent that does not react with Al and Mg substrates can be used toplate the desired plating metals directly onto the Mg or Al substrate.

Aluminum chloride (AlCl₃) is one example of a covalently bonded compounduseful as one component of a molten salt solvent. AlCl₃ occurs as thedimer (Al₂Cl₆) and will readily combine with almost any free chloride toform a tetrahedral aluminum tetrachloride anion (AlCl₄ ⁻). Thiscovalently bonded ion acts as a large monovalent ion, with the negativecharge dispersed over a large volume. The complex salt (such as NaAlCl₄)has a far lower melting point than the corresponding simple chlorides.The alkali metal tetrachloroaluminate complex salts (NaAlCl₄ and KAlCl₄)have been used as moderately high temperature solvents (for example at150-300° C.) for a variety of purposes, including electrochemistry,spectroscopy, and crystal growth.

Ambient temperature molten salts car also be formed from aluminumchloride. Table 1 lists some of the compounds capable of forming ambienttemperature molten salts when combined with aluminum chloride. All ofthe materials listed are ionic chlorides. With the exception of TMPAC,all have the positive charge delocalized to some degree through aπ-conjugated system over a large portion of the volume of the bulkycation. In all cases, the combination of a large cation, with a lowcharge density and a large anion, with a low charge density, leads to alow melting solid. The combination is an ionic liquid that actuallybehaves in some respects more like a molecular liquid. Unlike hightemperature molten salts, which tend to interact only throughnon-directional charge-charge interactions, these molten salts arehydrogen bonded liquids with the cations forming a water-like network.

TABLE 1 Compounds that Form Room Temperature Tetrachloroaluminate SaltsCompound Formula Abbr. 1-ethyl-3-methylimidazolium chloride C₆H₁₁N₂ClEMIM Trimethylphenylammonium chloride C₉H₁₄NCl TMPAC1-methyl-3-ethyl-imidazolium chloride C₆H₁₁N₂Cl MEIC1,3-dimethyl-imidazolium chloride C₅H₉N₂Cl 1-methyl-3-propyl-imidazoliumchloride C₇H₁₃N₂Cl 1-methyl-3-butyl-imidazolium chloride C₈H₁₅N₂Cl1,3-dibutyl-imidazolium chloride C₁₁H₂₁N₂Cl1,2-dimethyl-3-propyl-imidazolium chloride C₈H₁₅N₂Cl DMPrIClN-butylpyridinium chloride C₉H₁₄NCl BPC N-propylpyridinium chlorideC₈H₁₂NCl N-ethylpyridinium chloride C₇H₁₀NCl N-methylpyridinium chlorideC₆H₈NCl

Transition metals are more easily plated from molten salts than reactivemetals, since they are more easily reduced than the solvent, instead ofbeing part of the solvent. In molten salts, like in aqueous solutions,the most easily reduced species will be reduced and deposited (plated)first, with that species protecting the solvent from reduction until itis consumed. Some of the materials plated, together with the base usedin the solvent system used for plating are listed in Table 2.

TABLE 2 Metals and Alloys Plated from Room Temperature Molten SaltsElement Base Co MEIC Co-Al alloy MEIC Co BPC Co-Al alloy BPC Ni MEICNi-Al alloy MEIC Cu BPC Pd MEIC Au MEIC Ag MEIC Pb MEIC Al-Cr alloysTMPAC

Some of the elements in Table 2 were plated from acidic melts, othersfrom basic melts. A few have been plated from both acidic and basicmelts. The substrates used in these plating tests varied widely as well,with relatively refractory materials such as Pt and glassy carboncommon, as well as Al.

Since Al and Mg are generally more electroactive than the metals to bedeposited, it is likely that the material initially deposited will be analloy of Al or Mg and the metal being deposited. As seen in Table 2,these types of alloys have been observed for several elements (Co, Ni,and Cu). In those cases, the careful and continued deposition of thetransition metal leads to a pure transition metal surface. The elementswere determined to behave in accordance with their position in theelectromotive series and an examination of the appropriate binary phasediagram to identify potential for forming intermetallic phases.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the above recited features and advantages of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference to theembodiments thereof which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a front perspective view of a serpentine flow field from a PEMfuel cell.

FIG. 2 is a schematic cross-sectional view of the bipolar plate of FIG.1, showing the contact surfaces plated with precious metal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to light weight bipolar plates for use inelectrochemical devices, such as fuel cells. These bipolar plates aremade from light metals, such as Al and Mg. An Al or Mg substrate isplated with metals that provide corrosion resistance and high electricalconductivity to the resulting bipolar plate.

Aluminum and magnesium are lightweight alternatives to graphite bipolarplates, because Al and Mg provide much greater electrical conductivitythan graphite while having similar densities to graphite. Al and Mg canalso be machined easily with better control of shape and thickness thancan be achieved with graphite. Al and Mg may also be formed by diecasting, which increases manufacturing throughput, and consequentlylowers production costs.

TABLE 3 Die Casting Alloy Compositions ASTM No. B380.0 Az91D Element wt% Wt % Al 82.75-85.75 8.3-9.7 Mg 0.10 88.71-91.63 Cu 3.0-4.0 0.03 Fe 1.30.005 Si 7.5-9.5 0.05 Mn 0.50 0.15-0.50 Zn 1.0 0.35-1.00 Ni 0.50 0.002Sn 0.35 —

Al and Mg with no coating are subject to corrosion. Therefore, thesurfaces of these metals are preferably coated with at least one layerof a metal that will impart corrosion resistance without interferingwith the electrical or electronic conductivity of the plate. Variousplating techniques may be used, such as, displacement Ni and electrolessNi for the first layer of plated metal. The corrosion resistance ofelectroless nickel-phosphorous alloys is well known and are widely usedfor corrosion protection of Al and mild steel. Since displacement Ni andelectroless Ni deposits are uniform in thickness over all surfaces,irregular shapes can be protected from corrosion in areas whereelectroplated deposits would be thin or totally lacking, clue to currentdistribution limitations. Electroless nickel-phosphorous, with anamorphous structure, will also help seal the porosity on the surfaces ofthe Al or Mg substrate.

The rate of the Ni displacement reaction should be carefully controlledby controlling the temperature of the bath and I-he concentration of thenickel solution. A slow displacement reaction produces fine-grained Niwith uniform surface coverage and strong adherence to Al substrates.Careful selection of the complexing agent, which is typically amulti-functional or hydroxylated carboxylic acid with a short carbonchain, (e.g., DL-malic acid, gluconic acid, citric acid, etc.) and thebath temperature, which is typically between about 50 and about 70° C.,Rives rise to the desired Ni displacement reaction rate, which should beless than 0.3 μm/hr.

Electroless Ni coatings deposited at a temperatures less than about 80°C. can exhibit residual stress. The pH of the electroless platingsolution has a strong effect on the magnitude of the residual stress.However, known annealing procedures can be used to relieve the stress inelectroless Ni deposits. One such annealing procedure involves annealingat about 120° C. for 1 hour in an inert gas environment.

Table 2, above, shows a list of elements which have been plated fromroom temperature molten chloride salts. In general, the same processesare observed with the corresponding bromide salts, but with noadvantage. Some of these elements were plated from acidic (AlCl₃ rich,relative to the nitrogen-containing base) melts, others from basic(nitrogen-containing base rich) melts. A few have been plated from bothacidic and basic melts. The substrates used in these plating testsvaried widely as well, with relatively refractory materials such as Ptand glassy carbon common, as well as Al.

Another embodiment of the invention involves the direct plating ofprecious metals onto Al and Mg from a room temperature molten saltsolvent. This avoids the problems associated with the Al or Mg reactingwith the water in the aqueous solvent. Alternatively, a base metalundercoat from room temperature molten salt solvent may be depositedfirst, followed by plating a precious metal top coat. The precious metaltop coat can be deposited from either a second room temperature moltensalt solution or an aqueous solution.

One advantage of applying two separate coats is the ability to tailorthe coating to the location on the plate. Specifically, when the basemetal layer is sufficient to protect the light metal from corrosiveattack, but does not furnish good electrical contact, a thin layer ofprecious metal can be applied only to the contact surfaces. This iseasily accomplished simply by plating without making any effort to platethe less accessible parts of the bipolar plate. Some of the metal isdeposited in the channels, but most of the metal will remain on the topsurfaces as shown in FIG. 2.

As in the aqueous system, Ru and Pd, which are stable in the PEM fuelcell environment, are the elements of choice for the non-aqueous platingsystem. Both are less expensive than Au on a weight basis and both areone third less dense, thus covering a larger area per unit of materialthan Au. In addition to the material cost savings, Pd layers are lessporous than Au layers, offering better protection from corrosive attack.

Depositing the top coat using conventional aqueous methods has a numberof advantages. Most obviously, it permits the use of a wide range ofexisting, commercially available chemistries. In addition, it allows forthe inclusion of PTFE in the final layer, which provides the waterrepellent properties that are useful in certain fuel cell operations.

The following examples illustrate some of the preferred embodiments ofthe present invention.

EXAMPLE 1

An aqueous plating process was used to deposit strongly adherent metalfilms on the surfaces of Al alloys (Al 2024-T3 and Al 7075-T6) in theform of coupons (3″×1″×{fraction (1/16)}″). The overall coating processconsisted of four main steps: (1) surface pretreatment, (2) displacementNi deposition, (3) electroless Ni deposition, and (4) Au electroplating.In the surface pretreatment step, the Al alloy coupons were degreasedand most of the surface oxide film was removed or etched. The degreasingand etching steps were carried out in an inert gas (Ar) environment freefrom oxygen.

The pretreatment procedure consisted of the following steps: 1)Sonicating the Al coupon in industrial detergent for 5 min.; 2) Rinsingthe Al coupon thoroughly with deionized water; 3) Drying and sandblasting the Al coupon; 4) Sonicating the Al coupon in deionized water;5) Degreasing the coupon in 10% NaOH for 30 sec., 6) Rinsing the Alcoupon thoroughly with deionized water; 7) Etching the Al coupon in 60%HNO₃+100 g/L NH₄F.HF for 30 seconds; and 8) Rinsing the Al couponthoroughly with deionized water. After the surface pretreatment step,All alloy coupons were rinsed thoroughly with deionized water andexhibited shiny metallic surfaces.

Surface pretreated coupons were quickly immersed in an Ar stirred Nidisplacement bath. The composition and operating conditions of the Nidisplacement bath are given in Table 7. DL-malic acid was used as thecomplexing agent and the Ni displacement step was allowed to proceed forapproximately 20 min. In this step, Ni metal plated onto freshly exposedAl metal surfaces in an oxygen-free aqueous solution via displacementfrom solution by dissolving Al atoms. The Ni displacement process givesrise to the deposition of at most a few monolayers of Ni on the surfacesof Al substrates since it is an exponentially decelerating andself-limiting process that coats exposed Al metal, but ceases once theAl surface is covered with a thin layer of Ni metal.

TABLE 7 Composition of Nickel Displacement Bath and Plating ConditionsNiSO₄.6 H₂O 0.05M DL-Malic acid 0.20M Temperature 60-70° C. pH 8.0-9.0Agitation Ar

After removing the Al alloy coupons from the Ni displacement bath, theywere rinsed thoroughly with deionized water and then immersed in anelectroless Ni plating bath. The composition of the electroless Ni bathand the conditions used are given in Table 8. The thin film of Nideposited from the Ni displacement bath provided catalytic sites forinitiation of electroless Ni deposition. The electroless Ni depositionprocess initially proceeded very vigorously as evidenced by strong gasevolution from the coupon surfaces. In this bath, hypophosphite aniorisserved as a reducing agent, reducing Ni cations and depositing Ni metalon substrates with the concomitant generation of hydrogen gas. Theagitation produced by the gas helped to enhance the mass transfer ofionic species and, thus, improve the smoothness of electroless Nideposits. This process was allowed to proceed for 1-2 hours. The platedNi was observed to give a bright grayish color to the surfaces of thecoupons. From microscopic examinations, the combined thickness ofdisplacement Ni and electroless Ni deposits was of the order of 7 μm. Oncompleting the electroless Ni deposition process, coupons were sonicatedin deionized water for 10 minutes, thoroughly rinsed with deionizedwater, and dried in air.

In a final coating step, the Al alloy coupons were plated with a layerof Au to enhance corrosion resistance. Prior to applying anelectroplated Au layer, the surfaces of the Ni deposits were pretreatedeither with intrusion Au or with a Au strike to facilitateelectroplating of Au and improve the adhesion of pure Au deposits to theAl substrate. The Au strike involved the use of a commercial platingsolution (EAS solution, available from Engelhard) and was carried out at80° C. using an applied current density of 50 mA cm⁻² for 40 seconds.For immersion Au plating, a commercial solution (ATOMEX®,

TABLE 8 Composition of Electroless Nickel Bath and Plating ConditionsNiSO₄.6H₂O 0.05M glycine 0.15M NaH₂PO₂.6H₂O 0.30M Temperature 70-80° C.pH 9.5-10 Agitation Ar

available from Engelhard) was used. The conditions included atemperature of 70-90° C., at a pH of 5.0-5.4, and for a plating time of10-15 minutes. The Au electroplating was carried out using a commercialAu plating solution (E-56; also available from Engelhard). An appliedcurrent density of 50 mA cm⁻² was used for 20-30 minutes at 70-80° C. Abright, reflective surface was obtained after electroplating Au on thesurfaces of the coupons. Microscopic examination of cross sectionedcoupons revealed that electrodeposited Au layers were compact inappearance and had thicknesses of the order of 20 μm.

EXAMPLE 2

This example illustrates the suitability of plated aluminum for fuelcell use.

To determine the suitability of metal-coated Al alloys for use asbipolar plates in PEM fuel cells, a number of tests were carried out onthe coated coupons of Example 1. The tape peel test and surfaceelectrical resistance measurements indicated excellent adhesion betweendeposited metal layers and Al alloy substrates and the successfulremoval of electrically resistive Al oxide films. A thermal cyclingstress test was carried out by heating specimens in an oven at 100° C.for 1 hour and then quenching the specimens in room temperature (22-25°C.) ditlul:e aqueous nitric acid (pH 2). After 200 quenching cycles,there was no evidence of blistering, debonding, or other deteriorationof plated coupons.

The coated coupons were subjected to corrosion tests in aerated diluteaqueous nitric acid solutions (pH 2). The coupons were half immersed inthe acid solution and half exposed to the air to simulate the conditionsthat would be encountered in the cathode of a PEM fuel cell. The firsttest was carried out at room temperature under open circuit conditions,with blister formation observed at the sample/solution/air three-phaseinterface after approximately 200 hours of exposure. The second testinvolved accelerated corrosion under potential polarization using athree-electrode system more closely mimicking the environment in thecathode of a PEM fuel cell. The metal-coated coupons were partiallyimmersed and held at a potential of +1.0 V (NHE) in an acid solutionmaintained at 60° C. Under these conditions, the best metal-coatedcoupons failed with the formation of small blisters after approximately100 hours of exposure. Unplated Al alloy coupons failed almost instantlyunder these test conditions.

Careful examination of the failure sites indicated that the blisterswere the result of pinhole flaws in the metal coating. These flaws weretoo small to be observed, but large enough to permit the penetration ofH₃O⁺ and water molecules, which could then attack the Al and slowlyundercut the plated metal until a blister became apparent. These flawswere suspected to have arisen from holidays in the initial Nidisplacement coating and may be due to inclusions or inhomogeneities inthe Al alloy substrate. Each of the subsequent plated layers increasedthe overall coating thickness, and probably partially covered theinitial flaws, but none of the additional layers was perfect, so thateven the smallest flaw could lead to eventual failure.

The results described above show that the plating processes employedsignificantly improved the corrosion resistance of commerciallyavailable Al alloys. It must be pointed out that these are acceleratedtests and the test solution used, namely, aerated aqueous nitric acidhaving a pH of 2, is much more aggressive than the environment thatwould be encountered in the air fed cathode compartment of a PEM fuelcell.

EXAMPLE 3

This example illustrates plating from a room temperature molten saltsolution.

An aluminum coupon is immersed in a molten salt bath consisting of a 1:1mole ratio mixture of MEIC and AlCl₃ at 25° C. in an argon filled glovebox. A platinum counter electrode is also placed in the bath and asource of electrical current attached to both the coupon and theplatinum electrode. The aluminum coupon is polarized relative to theplatinum electrode in such a manner that the flow of electrical currentwill cause the dissolution of a small portion of the aluminum from thesurface, and the current switched on. This process serves to remove thecoating of aluminum oxide normally present on the surface of aluminum.

A 3 mole percent solution of NiCl₂ in MEIC—AlCl₃ is also prepared in theargon filled glove box. The cleaned aluminum coupon is placed in thenickel-containing bath, along with a nickel wire counter electrode. Apotential is imposed on the coupon and the nickel wire so as to causenickel from the solution to deposit on the surface of the aluminum, andnickel from the wire to dissolve into the molten salt. As the flow ofcurrent continues, an initial layer of nickel-aluminum alloy is formedon the surface of the aluminum coupon. This alloy serves as the base forthe deposition of a layer of pure nickel, which occurs as the depositioncontinues.

After the deposition is terminated, the coupon, now with a layer ofnickel on its surface is washed with acetonitrile to remove any tracesof the plating bath. At this point the coupon is ready to be removedfrom the argon atmosphere and receive a top coat of ruthenium prior totesting for corrosion resistance.

While the foregoing is directed to the preferred embodiments of thepresent invention, other and further embodiments of the invention may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims which follow.

We claim:
 1. A bipolar plate comprising: a metal substrate selected fromMg, Al Mg alloys and Al alloys; a displacement Ni layer deposited on thesubstrate; an electroless metal layer selected from Ni, Pd, Co, Ru, Au,Pt and mixtures thereof deposited on the displacement Ni layer; and afirst electroplated metal layer selected from Ni, Ru, Pd, Au, Pt, andmixtures thereof deposited on the electroless metal layer; and a secondelectroplated metal layer deposited over the first electroplated metallayer, wherein the second electroplated metal layer is selected from Pt,Au, Ir, Pd, Ru, and mixtures thereof, and wherein the secondelectroplated metal layer further comprises PTFE.
 2. A bipolar platecomprising: a metal substrate selected from Mg, Al, Mg alloys and Alalloys; and a metal layer selected from Co, Pt, Au, Ir, Pd, Ru andmixtures thereof deposited on the substrate, wherein the metal layer isdeposited on the substrate from a non-aqueous solution.
 3. A process formaking lightweight bipolar plate comprising: deoxidizing a metalsubstrate selected from Mg, Al, Mg alloys and Al alloys; and depositinga first metal layer on the metal substrate using a non-aqueous solutionselected from a molten salt having a cation containing at least onetetravalent nitrogen atom and a halogenated aluminum species.
 4. Theprocess of claim 3, further comprising depositing a second metal layeron the first metal layer, wherein the second metal layer is depositedusing a non-aqueous solution.
 5. The process of claim 3, wherein thefirst metal layer contains a metal selected from Pt, Au, Ir, Pd, Ru andmixtures thereof.
 6. The process of claim 3, wherein the first metallayer contains a metal selected from Ni, Pd, Co, Ru and mixturesthereof.
 7. The process of claim 6, further comprising depositing asecond metal layer on the first metal layer wherein the second metallayer is deposited using a non-aqueous solution, wherein the first metallayer contains a metal selected from Ni, Pd, Co, Ru and mixturesthereof, and the second metal layer contains a metal selected from Pt,Au, Ir, Pd, Ru and mixtures thereof.
 8. The process of claim 3, furthercomprising depositing a second metal layer on the first metal layer,wherein the second metal layer is deposited using a conventional aqueousplating solution.
 9. The process of claim 3, further comprisingannealing the first metal layer.
 10. A bipolar plate comprising: a metalsubstrate selected from Mg, Al, Mg alloys and Al alloys; a displacementNi layer deposited on the substrate; an electroless metal layerdeposited on the displacement Ni layer; a first electroplated metallayer deposited on the electroless metal layer; and a secondelectroplated metal layer comprising polytetrafluoroethylene depositedon the first electroplated metal layer.
 11. The bipolar plate of claim10, wherein the electroless metal layer is selected from Ni, Pd, Co, Ru,Au, Pt and mixtures thereof.
 12. The bipolar plate of claim 10, whereinthe first electroplated metal layer is selected from Ni, Co, Ru, Pd, Au,Pt, and mixtures thereof.
 13. The bipolar plate of claim 10, wherein thesecond electroplated metal layer is selected from Pt, Au, Ir, Pd, Ru,and mixtures thereof.
 14. A process for making a lightweight bipolarplate comprising: deoxidizing a metal substrate selected from Mg, Al, Mgalloys and Al alloys; and depositing a first metal layer on the metalsubstrate using a non-aqueous molten salt solution.
 15. The process ofclaim 14, wherein the non-aqueous molten salt solution comprises acation containing at least one tetravalent nitrogen atom and ahalogenated aluminum species.
 16. A lightweight bipolar plate,comprising: a light metal selected from aluminum, magnesium, aluminumalloys and magnesium alloys; wherein the surface of the light metal iscoated with at least one layer of a metal, and wherein the at least onelayer of a metal comprises an electroplated metal-polytetraluoroethylenecomposite coating.
 17. The bipolar plate of claim 16, wherein theelectroplated metal is selected from ruthenium, palladium and gold.